Method and apparatus for low energy electron enhanced etching of substrates in an AC or DC plasma environment

ABSTRACT

An apparatus for low-damage, anisotropic etching of substrates having the substrate mounted upon a mechanical support located within an ac or dc plasma reactor. The mechanical support is independent of the plasma reactor generating apparatus and capable of being electrically biased. The substrate is subjected to a plasma of low-energy electrons and a species reactive with the substrate. An additional structure capable of being electrically biased can be placed within the plasma to control further the extraction or retardation of particles from the plasma.

RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 09/855,972,filed May 15, 2001, now U.S. Pat. No. 6,852,195, which is a division ofU.S. application Ser. No. 08/932,025, entitled “Method And Apparatus ForLow Energy Electron Enhanced Etching of Substrates in an AC or DC PlasmaEnvironment”, filed Sep. 17, 1997 now U.S. Pat. No. 6,258,287, whichclaims priority to and the benefit of the filing date of ProvisionalPatent Application Ser. Nos. 60/026,985, filed Sep. 20, 1996, entitled“APPARATUS AND PROCESS FOR LOW-DAMAGE DRY ETCHING OF INSULATORS BY LOWENERGY ELECTRON ENHANCED ETCHING IN A DC PLASMA”; 60/026,587, filed Sep.20, 1996, entitled “APPARATUS AND PROCESS FOR LOW-DAMAGE DRY ETCHING OFINSULATORS BY LOW ENERGY ELECTRON ENHANCED ETCHING TN AN AC PLASMA”; andis a Continuation-In-Part of U.S. patent application Ser. No.08/705,902, filed on Aug. 28, 1996 now U.S. Pat. No. 5,882,538 entitled“METHOD AND APPARATUS FOR LOW ENERGY ELECTRON ENHANCED ETCHING OFSUBSTRATES”.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Grant No.DMR-9202879, awarded by the National Science Foundation of the U.S. TheGovernment has certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates generally to the preparation of etchedsubstrates. More particularly, the present invention relates to animproved process for low-damage, anisotropic etching of substrates suchas semiconductors and insulators, and improved anisotropically etchedsubstrates.

BACKGROUND OF THE INVENTION

Dry etching is an absolutely critical process in the fabrication of allmicrometer and nanometer scale features on high-speed electronic andopto-electronic devices. In brief, the fabrication of such chips anddevices involves the following process. A substrate of somesemiconductor or metal is selected and a pattern is laid down over it,the pattern having open areas in it. The overlying structure containingthe pattern is sometimes called a mask. Etching chemistry is thenperformed through the open areas, which means that in effect some of theunderlying material exposed through the open areas is dissolved away sothat the pattern is transferred into the underlying layer(s). Then, themask is stripped away and what is left behind is the original substrate,but now the pattern has been transferred into it. The process is similarto silk screening or stamping a pattern into material. The resultingpattern has a three dimensional structure.

In the early days of integrated circuit fabrication, most etching wasdone using a wet chemical process that is quite similar to conventionalphotography. For example, to etch an array of grooves in a siliconwafer, the wafer is first placed in a high temperature, oxidizingenvironment and a layer of silicon dioxide is grown on the top surfaceof the wafer. Then, the oxidized wafer is covered with a thinphotosensitive layer of gelatinous organic material called a“photoresist”. Next, a piece of material analogous to a photographicnegative, called a “photomask”, is placed over the photoresist.Ultraviolet light is shined through openings in this photomask. Theultraviolet light changes the solubility of the photoresist. Thus, areasof photoresist that have been illuminated with the ultraviolet lightdisplay a different solubility than areas which have not been exposed tothe light. Finally, a solvent is used which dissolves away only theareas of the photoresist, which have had their solubility increased bythe ultraviolet light. At this point, the original pattern on thephotomask has been transferred to the photoresist layer. Some peoplerefer to this patterned photoresist layer as a “soft mask”.

Subsequently, a wet chemical, hydrofluoric acid (HF) dip is used todissolve away the silicon dioxide, which has been exposed through eachof the openings in the overlaying photoresist. Then, the photoresist isstripped off. At this point, it is apparent that the pattern originallyappearing on the photomask has been transferred to the silicon dioxidelayer overlying the silicon wafer. This patterned layer of silicondioxide is sometimes referred to as a “hard mask”.

Finally, the wafer is dipped into a caustic etch, such as potassiumhydroxide (KOH) which etches away the silicon exposed under the openingsin the hard mask. After stripping away the hard mask, the desiredsilicon wafer with the etched grooves remains.

As an alternative to following the hard masking step described abovewith a KOH silicon etch, ion implantation or high temperature diffusioncould optionally be used to place dopant atoms through the openings inthe hard mask.

Many other structural and chemical variations using the sorts of wetprocessing steps described above are possible and are well known tothose of skill in the art. In each case however, the correspondingprocess suffers from a problem that is inherent with the associatedetching using wet chemicals. In particular, at the same time that thewet chemistry is etching down into the wafer, it is also etchinglaterally under the mask. Indeed, this undesired lateral etching tendsto extend approximately as far as the desired vertical etching. Thistendency for wet chemical etching to proceed equally in all directionswithout prejudice is called “isotropic etching”.

Isotropic etching is adequate for making a line that is 20 microns widethrough a film that is 1 micron deep. The resulting inaccuracy in theedges of such a feature is a small percentage of the overall devicestructure; and therefore, it does not compromise performance. However,as smaller and smaller structures are fabricated, isotropic etching isinadequate. The industry is moving toward fabrication of structures withso called sub-micron features, which are essential for high speedcomputer chips, optical structures, and electronic and optoelectronicdevices. In other words, the accurate transfer of a pattern, which ishalf a micron wide into a material, which is half a micron thick,requires absolutely straight vertical sidewalls, or anisotropic etching.Isotropic etching is inadequate because the associated rounded undercutwould be a very high percentage of the active device material and woulddestroy its performance.

Presently, it is thought that the only way to get straight sidewalls isby a technique called reactive-ion etching (RIE). Rather than dippingthe device in wet chemicals, it is exposed to reactive gases in plasmas.Energetic ions formed in the plasma are accelerated in the normaldirection to the substrate where they enhance the etching chemistry atthe bottom of the open area defined by the mask and not on thesidewalls. Thus, straight sidewalls can be achieved with reactive-ionetching.

Reactive-ion etching provides anisotropic etching. However, the ions areheavy ions like argon or CF₃ ⁺ and are traveling at a few hundredelectron volts of kinetic energy. Thus, they carry enough momentum todisplace lattice atoms from their normal position. This damages thesurfaces and often, the optical and electrical properties of thesubstrate have been detrimentally changed.

Fabrication of ultra small electronic and optoelectronic devicesrequires dry etching processes that give high anisotropy, highselectivity between different materials, and minimal surface damage.Currently, ion enhanced plasma etching processes (e.g. reactive ionetching (RIE) and electron cyclotron enhanced RIE (ECR)) create highaspect ratio nanometer scale features; however, etch induced damage hasbecome increasingly troublesome as critical dimensions shrink. Tominimize etch damage, reactive species generated in the plasma shouldhave energies larger than the activation energy of the etch reaction (afraction of an eV), but less than the energy required for atomicdisplacement (3 to 10 eV for III-V semiconductors). Given theselimitations, the ion energies available in reactive-ion (about 300 eV)and electron cyclotron resonance plasma etching (about 50 eV) are notideally suited for fabricating nanometer scale devices.

Placing the sample to be etched on the anode within a DC plasmaenvironment is one way to ensure precise control over the anisotropicetching process while minimizing damage to the substrate and isdescribed in commonly assigned U.S. Pat. No. 5,882,538, filed Aug. 28,1996 and titled “METHOD AND APPARATUS FOR LOW ENERGY ELECTRON ENHANCEDETCHING OF SUBSTRATES”. This technique is called Low Energy ElectronEnhanced Etching (LE4, for convenience), and operates by placing theetching substrate on the anode of a DC glow discharge. This method workswell for conducting and semi-conducting substrates, but is inherentlyproblematic for etching non-conducting substrates such as insulatorsbecause, in the aforementioned method, the substrate sample isphysically and electrically connected to the anode in the plasma, thusbecoming a conducting element of the electrical circuit within theplasma. An insulator, by definition does not efficiently conductelectrical current, therefore, placing an insulating substrate on theanode will impede the electrical flow and will be an inefficient andnearly impossible way to etch a non-conducting substrate.

What is needed and was apparently not available until the presentlydescribed invention, is a method of etching that eliminates the damageinflicted by reactive-ion etching, achieves anisotropic etching, and isindeed applicable to all forms of substrates, including insulatingsubstrates. Furthermore, it is desirable to have additional control overthe etching process. Specifically, it is desirable to have the abilityto control more precisely the flux, or rate per unit area of particlesbeing imparted to the sample, and the energy that the particles impart.

A method of electron-impact-induced anisotropic etching of silicon (Si)by hydrogen is discussed in a 1982 article by S. Vepcek and F.-A.Sarott, “Electron-Impact-Induced Anisotropic Etching of Silicon byHydrogen”, Plasma Chemistry and Plasma Processing, Vol. 2, No. 3, p.233. The authors discuss successful etch rates of up to 1,000 Å/min withlittle surface roughness at low temperatures. At higher temperatures arougher pattern was seen. While their exact methodology is unclear, theauthors apparently used an apparatus described in a previous publicationby A. P. Webb and S. Vepcek, “Reactivity of Solid Silicon With HydrogenUnder Conditions of a Low Pressure Plasma”, Chemical Physics Letters,Vol. 62, No. 1, p. 173 (1978). That publication describes an apparatusincluding a DC glow discharge device with the sample immersed in thepositive column. The cathode was a standard hot cathode (heated tobetween 1500-2000 K) having a tungsten filament coated with thoriumoxide. While this technique apparently worked for etching Si(111) withhydrogen, it would not work using other reactive gases such as oxygen,chlorine, and fluorine because the hot filament would be immediatelyconsumed. Furthermore, the apparatus described by Vepcek and Sarott iscumbersome.

Other experiments, reported by Gillis et al. in an article entitled“Low-Energy Electron Beam Enhanced Etching of Si(100)-(2×1) by MolecularHydrogen,” J. Vac. Sci. Technology B 10(6), November/December, p. 2729(1992), focused on flooding Si with low energy electrons (200-1000 eV)produced by an electron gun. The authors reported etching at a rate ofabout 100 Å/min with low damage to the Si surface. Other papers by thepresent inventors are: “Low Energy Electron—Enhanced Etching of Si(100)in Hydrogen/Helium Direct—Current Plasma”, (Gillis et al., Appl. Phys.Lett., Vol 66(19), p. 2475 (1995)); “Low Energy Electron—EnhancedEtching of GaAs(100) in a Chlorine/Hydrogen DC Plasma” (Gillis et al.,Appl. Phys. Lett., Vol. 68(16), p. 2255 (1996)); “Low EnergyElectron-Enhanced Etching of GaN in a Hydrogen DC Plasma” J.Electrochem. Soc., 143, L251 (1996); and “Highly Anisotropic,Ultra-smooth Patterning of GaN/SuiC by Low Energy Electron EnhancedEtching in a DC Plasma” (Gillis et al., J. Electr. Mat. 26, 301-305(1997)). These publications are incorporated herein by reference, intheir entireties.

H. Watanabe and S. Matsui, writing in Applied Physics Letters, Volume61, 1992, pp. 3011-3013, describe a related approach to achieve aprocess they call Electron Beam (EB)-assisted dry etching. They useporous grids to extract a “shower” of electrons from an ECR plasma anddirect it toward a substrate. However, they operate the ECR source onargon gas, and then insert a separate gas ring nozzle between extractiongrids and substrate. This gas ring, resembling somewhat an ordinary gasburner on a kitchen range, distributes the reactive gas (chlorine, intheir case) over the etching substrate. They apply only a DC bias to thesubstrate, to increase electron collection efficiency and energy. Theyreport results only for semiconductor substrates, not insulators.

SUMMARY OF THE INVENTION

The present invention involves low energy electron enhanced etching(LE4), as opposed to reactive ion enhanced etching, and comprises animprovement thereupon by permitting the effective anisotropic etching ofall substrates including conducting, semi-conducting and insulatingsubstrates. The process and apparatus give straight sidewalls, and itdoes not damage the substrate. In contrast to the above-describedreactive ions, the low energy electrons that are used in the presentinvention travel at less than about 100 electron volts (eV) kineticenergy (KE), preferably at less than about 20 eV. The mass of electronsis many orders of magnitude smaller than the mass of ions and theelectrons carry essentially no momentum to the surface. Therefore, theydo not damage the surface.

Furthermore, the present invention allows significantly enhanced controlover the etching process by permitting variations in the flux, or rate,of electrons and neutral specie particles imparted to a sample substrateand the energy of arrival of those particles imparted to a samplesubstrate. This is done by establishing a local electrical field in thevicinity of the sample with respect to the plasma, thus any chargedspecie of particle that feels that potential will respond enabling theextraction or retardation of particles from the plasma. Because this isdone in the vicinity of the sample, the present invention allows precisecontrol over the flux and energy parameters of the particles arriving atthe sample.

The present invention provides a low damage alternative to ion enhancedprocesses, using low energy electron enhanced etching (LE4), in whichthe substrate is supported on a structural member within the plasmafield, and in an alternate embodiment, a conducting substrate issupported by an anode of a dc plasma reactor. In all but the embodimenthaving the sample supported by the anode of a dc plasma reactor, theplasma field may be a DC plasma or an AC plasma.

The energy of electrons and negative ions arriving at the substrate onthe anode of a DC discharge is limited to a value not greater than theionization potential of the reaction gas;

-   -   energies above this limit are effectively dissipated by        inelastic collisions in the gas phase. No such fundamental limit        is imposed on the positive ions produced in RF and microwave        plasmas. For dc plasma reactors a voltage of about 0.5-2 kV is        applied between the cathode and the anode, generating a glow        discharge in which electrons having a kinetic energy of less        than about 100 eV, or, preferably less than about 20 eV arrive        at the anode.

In one aspect, the invention involves a process for low-damage,anisotropic etching of a substrate that includes the steps of placingthe substrate on a mechanical structure designed to support the sampleto be etched within a plasma reactor and subjecting the substrate to aplasma including low energy electrons and a gaseous species that isreactive with the substrate. The substrate can be a Group IVsemiconductor, a Group III-V semiconductor, a Group II-VI semiconductor,an oxide, a nitride, a metal, an alloy or mixture of the foregoing, oran insulating substrate. The reactive species can be any that reactswith the substrate and that volatilizes within the temperature and flowof the device. Typical reactive species to be used are hydrogen,halogens, interhalogen compounds, hydrogen halides, and volatile organiccompounds. The concepts of the present invention are equally applicableto an ac plasma or a dc plasma. Three embodiments of the presentinvention will be discussed.

It is important to note that in the case of a plasma reactor that useselectrical conductors such as an anode and a cathode to generate theplasma flow, in all but one variation, the mechanical support is notused as an electrode to generate or maintain the plasma flow, but in afirst preferred embodiment, merely as a mechanical support for thesample to be etched, and in subsequent embodiments, not as an electrodeinvolved in the generation or maintenance of the plasma, but one whichboth mechanically supports the sample and also imparts an electricalbias to the sample resting thereon.

In a first preferred embodiment in which no external voltage is appliedto the mechanical support, and thus the sample, the sample achieves a“floating potential”, which in the case of a dc plasma, is negativerelative to the “plasma potential” in the body of the plasma.

In a second preferred embodiment, and with respect to a dc plasmareactor, an external electrical bias, with respect to the plasmapotential, is applied to the mechanical support, and therefore to thesample. The external electrical bias applied to the mechanical supportcan be of a dc or an ac nature, or a combination of the two. To preventbuildup of a negative potential, which can eventually stop the etchingprocess, the sample must be periodically “discharged” or “neutralized”by reducing the external bias slightly below plasma potential. This willenable a sufficient number of positive ions to arrive from the plasma tocancel the accumulated negative charge, while keeping their energy toosmall to inflict ion bombardment damage. Thus, the substrate on themechanical support must be subjected to a modulated positive voltage.

In a third preferred embodiment an additional structure, for example agrid, or a permeable cap, capable of imparting an electrical potentialis placed within the plasma. This additional structure is one that iscapable of being electrically biased in similar fashion to that of themechanical support used to support the sample. This additional structureis typically placed within the plasma in close proximity to the sampleto be etched. By varying the electrical bias to this structure, furthercontrol over the flux and energy of charged species, i.e., electrons andions, being imparted to the sample are achieved. When the plasma sourceis biased negative relative to these structures, a broad beam or streamof electrons will be extracted from the plasma toward the substrate. Theenergy of the electrons can be controlled by the magnitude of the dcbias on the ac source.

In a variation of this third embodiment, for etching conductingsubstrates, the sample is placed upon the anode of a dc plasma reactoras described in commonly assigned U.S. Pat. No. 5,882,538, filed Aug.28, 1996 and titled “METHOD AND APPARATUS FOR LOW ENERGY ELECTRONENHANCED ETCHING OF SUBSTRATES”, however as an improvement thereon, theadditional structure as described above is placed within the plasma inclose proximity to the sample, thus allowing improved precise controlover the flux and energy of charged species being imparted to thesample.

The process for the present invention with respect to an ac plasmareactor is similar to that described with respect to a dc plasmareactor. Variations necessary because of physical differences betweenvarious reactor configurations will not be discussed in detail. Themechanical support and the additional structure are equally applicableto both ac and dc plasma reactors. Furthermore, the benefits of thepresent invention are equally applicable to various ac plasma reactorconfigurations such as resonant microwave cavity reactors andinductively coupled plasma reactors.

In the second preferred embodiment as discussed above and with respectto an ac plasma reactor, an external electrical bias, with respect tothe plasma potential, is applied to the mechanical support, andtherefore, to the sample resting thereon. The external electrical biasapplied to the mechanical support can be of a dc or an ac nature, or acombination of the two.

The present invention also includes an apparatus for performinganisotropic etching of a substrate. The etching of any substrate,whether conducting, non-conducting, or insulating can be accomplishedusing the concepts of the present invention.

The apparatus includes any chamber as is known by those skilled in theart for generating a plasma. The apparatus used to practice the presentinvention is similar whether used within a dc or an ac plasma. Withinthe plasma chamber is placed a mechanical structure for physicallysupporting the sample to be etched. It is important to note that in allbut one possible configuration, in the case of a plasma reactor thatuses electrical conductors such as an anode and a cathode to generatethe plasma flow, the mechanical support is not used as an electrode togenerate or maintain the plasma flow, but in a first preferredembodiment, merely as a mechanical support for the sample to be etched,and in subsequent embodiments, not as an electrode involved in thegeneration or maintenance of the plasma, but one which imparts anelectrical bias to the sample resting thereon.

In a first preferred embodiment, i.e., one having no external voltageapplied to the mechanical support, and thus the sample, the sample isheld by the mechanical support in the plasma and allowed to achieve itsfloating potential.

In a second preferred embodiment, the mechanical support is capable ofbeing electrically biased with respect to the plasma potential. Thisallows the mechanical support to impart the electrical bias to thesample resting thereon. The external electrical bias applied to themechanical support can be of a dc or an ac nature, or a combination ofthe two.

An advantage of the present invention is that low damage sub-micronanisotropic etching of a conducting, semi-conducting, or insulatingsubstrate is achieved.

Another advantage of the present invention is that it is applicable to avariety of substrates using a variety of reactive species.

Another advantage of the present invention is that it allows an improveddegree of control over the flux and energy of charged species beingimparted to a substrate.

Another advantage of the present invention is that by moving the sampleoff the anode, electric current no longer has to pass directly throughthe sample, thus avoiding any charging damage that may occur if thesample is located at an electrode.

Another advantage of the method and apparatus of the present inventionis that the apparatus is fairly simple to assemble and operate.

Another advantage of the method and apparatus of the present inventionis that the permeable wall hollow cathode can generate a higher flux oflow energy electrons, at a lower pressure.

Other features and advantages of the method and apparatus of the presentinvention will become apparent to one with skill in the art uponexamination of the drawings and the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be better understood with reference to thefollowing drawings. The drawings are not necessarily to scale, emphasisinstead being placed upon clearly illustrating principles of the presentinvention.

FIG. 1 is a schematic view of a plasma reactor apparatus employing thefeatures of the present invention;

FIG. 2 is a schematic view of a dc plasma reactor illustrating anembodiment of the present invention;

FIG. 3 is a schematic view of a dc plasma reactor illustrating threepreferred embodiments of the present invention;

FIG. 4 is a schematic view of an illustrative ac plasma reactorillustrating three preferred embodiments of the present invention;

FIG. 5 is a detail schematic view of a sample support apparatus of thepresent invention; and

FIG. 5 a is a detail schematic view of an alternate embodiment of thesample support apparatus of FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the foregoing description of the preferred embodiments, the presentinvention is applicable equally both to a dc plasma reactor and an acplasma reactor. Variations necessary because of physical differencesbetween various reactor configurations are contemplated and will not bediscussed in detail. Furthermore, the benefits of the present inventionare equally applicable to various ac plasma reactor configurations, forexample, resonant microwave cavity reactors and inductively coupledplasma reactors.

In all embodiments that use conductors to generate the low energyelectrons desired to form the plasma flux, the cold cathode can be ahollow cathode formed with permeable, meshed, or perforated, generallyreferred to as permeable, walls rather than the typical solid walls. Thecathode may be cylindrically shaped with a sidewall of a permeableconductive material, such as stainless steel mesh, and having one endthat is open or closed and an open end. The cathode is connected to acathode mounting post and to the power supply. The cathode may comprisea plurality of nested sidewalls, each connected to the power supply. Theuse of this cathode allows the generation of a large flux of low energyelectrons at low pressure and temperature. A cathode made in accordancewith that described is described in commonly assigned U.S. Pat. No.5,917,285, filed Jul. 23, 1997, titled “APPARATUS AND METHOD FORREDUCING OPERATING VOLTAGE IN GAS DISCHARGE DEVICES”.

A preferred embodiment of an etching apparatus suitable for practicingthe concepts of the present invention, referred to generally as 10, isillustrated in FIG. 1. The apparatus includes a plasma chamber 11suitable for striking, maintaining, and containing a plasma as known bythose skilled in the art. Within plasma chamber 11 reside variousspecies of particles including positively charged ions 34, neutralspecies 36 and negatively charged electrons 37. The plasma generatingapparatus itself includes various means for generating low energyelectrons, maintaining a vacuum, introducing the charged and neutralspecies and monitoring and regulating the plasma process as is known inthe art, and has been omitted from FIG. 1 for clarity.

DC plasma reactors are ones in which a dc power source is used to strikethe plasma flow. AC plasma reactors are ones in which an ac power sourceis used to strike the plasma flow. AC plasma reactors can be forexample, conventional in which electrodes (an anode and a cathode) areemployed to generate the plasma flow, or electrodeless plasma generatorssuch as a resonant microwave cavity or an inductively coupled plasmagenerator. These and other plasma generating sources known to thoseskilled in the art are capable of being employed to practice theconcepts of the present invention, and indeed, the present invention canbe used in conjunction with any dc or ac plasma reactor with similarbenefit.

As shown in FIG. 1, apparatus 10 includes sample support apparatus 80which includes, among other items, mechanical support 12 designed tosupport substrate sample 16 in a suitable location within plasma chamber11. Sample 16 can be a conducting, semi-conducting, or an insulatingsubstrate.

Electrically connected to mechanical support 12 through connection 18 isAC power source 19. Connected to AC power source 19 on connection 22 isDC power source 26, which in turn is connected on line 29 to ground 35.

In a first preferred embodiment of the present invention, mechanicalsupport 12 is used to mechanically hold sample 16 in a suitable locationwithin plasma chamber 11, with AC power source 19 and DC power source 26inactive. In this embodiment, no external voltage is applied tomechanical support 12, and thus sample 16 achieves a “floatingpotential”, which in the case of both a dc plasma and an ac plasma isnegative relative to the “plasma potential” in the body of the plasma.In the absence of voltage applied to mechanical support 12, thedifference between floating and plasma potential naturally attracts ions34 from the plasma to sample 16. In the body of the plasma, the densityof electrons and ions is on average equal. Therefore in the absence ofany external electric fields, or in the vicinity of a perturbation, theplasma is at an equipotential. This is usually called the plasmapotential and is sometimes referred to as the space potential. Anelectrically isolated body, such as sample 16, placed in the plasma willinitially be bombarded by electrons and ions. The current density ofelectrons is much larger than that of the ions and so very quickly thebody will collect an excess of negative charge. This repels electronsand attracts ions. The electron flux is decreased so that it justbalances the ion flux on the body. At this point the body has reached asteady state of excess charge, electric field and potential. Thepotential it has reached is called the floating potential, and since itrepels electrons it is less than the plasma potential. In the instanceof a dc plasma the floating and plasma potentials are negative withrespect to the grounded anode. In the absence of a reference potential(e.g. when the plasma is created by ac inductive coupling), only thedifference in the plasma and floating potential is meaningful.

Still referring to FIG. 1 and in a second preferred embodiment, anexternal electrical bias with respect to the plasma potential, isapplied to mechanical support 12, and therefore to sample 16. AC powersource 19 and DC power source 26 are used either individually orcollectively to impart either an AC, DC or a combination of AC and DCelectrical bias to mechanical support 12. Application of increasinglypositive external voltage to mechanical support 12 decreases the netcurrent, which goes to zero when the external voltage exactly matchesthe floating potential. At this point, the current of positive ions 34exactly matches and compensates the current of electrons 37. Increasingthe positive external voltage to values greater than the floatingpotential causes the net current from plasma to mechanical support 12 tobecome progressively negative; at external voltages more positive thanthe plasma potential, mechanical support 12 rejects positive ions 34 andattracts electrons 37.

Most of these arriving electrons 37 will collide with sample 16, give upsome of their kinetic energy to it, and be reflected back into the bodyof the plasma. Some of them will stick to the substrate surface, therebytransferring negative charge to it. If sample 16 is an insulator, itcannot drain off this surface charge, which could accumulatesufficiently to generate a negative potential capable of blocking thearrival of further electrons 37, thereby stopping the etching process.To prevent this, sample 16 must be periodically “discharged” or“neutralized” by reducing the external bias supplied through mechanicalsupport 12 slightly below plasma potential. This will enable asufficient number of positive ions 34 to arrive from the plasma tocancel the accumulated negative charge, while keeping their energy toosmall to inflict ion bombardment damage. Thus, the substrate sample 16on mechanical support 12 must be subjected to a modulated positivevoltage supplied on connection 18 to mechanical support 12. When thisexternal bias is “high,” LE4 occurs. When this external bias is “low,”the negative charge on the substrate is being neutralized. The magnitudeof the positive bias must be sufficient to reject positive ions 34 andalso large enough to give electrons 37 sufficient energy to overcome thethreshold for LE4. The frequency of bias pulse must be sufficient toprevent negative charge buildup (which would stop LE4) by dischargingsample 16 often enough. The requisite magnitude and frequency, as wellas the optimal waveform (e.g. square wave vs. sinusoidal) must bedetermined empirically for each combination of substrate and etchinggas.

Still referring to FIG. 1 and in a third preferred embodiment,additional structure 14, which can be a grid, or permeable cap, capableof imparting an electrical potential is placed within plasma chamber 11.Additional structure 14 can be fabricated in a manner similar to thatdescribed with respect to the cold cathode discussed above. In oneembodiment, additional structure can be a grid placed in close proximityto sample 16 and will be explained in further detail below with respectto sample support apparatus 80 illustrated in FIG. 5. In an additionalembodiment, insulating non-permeable ring 23 is added to mechanicalsupport 12 and surrounds sample 16. Ring 23 is joined to mechanicalsupport 12 and additional structure 14 in such a way as to preventelectrons and ions from penetrating joints 13 and 13 a. This arrangementensures that any electrons or ions that reach sample 16 must passthrough additional structure 14 and will be explained in further detailbelow with respect to sample support apparatus 80 illustrated in FIG. 5a. This additional structure 14 is one that is capable of beingelectrically biased in similar fashion to that of mechanical support 12used to support sample 16. Additional structure 14 is typically placedwithin the plasma in close proximity to sample 16 to be etched.Additional structure 14 is connected on connection 17 to AC power source21. AC power source 21 is connected on connection 24 to DC power source28, which is connected on connection 31 to ground 32. By varying theelectrical bias to additional structure 14, further control over theflux and energy of charged species, i.e., electrons 37 and ions 34,being imparted to sample 16 is achieved. Specifically, the extractionand retardation of electrons 37 and ions 34 from the plasma to sample 16are controlled in order to further enhance the quality of the etchingimparted to sample 16. Similar to the electrical bias capable of beingapplied to mechanical support 12 holding sample 16, the electrical biasapplied to additional structure 14 by AC power source 21 and DC powersource 28 can be of an ac or dc nature, or a combination of the two.

In this third preferred embodiment, electrically neutral reactivespecies 36 will form reactive beams or streams collinear with electrons37. This combination of low energy electrons 37 and reactive species 36arriving at the surface will accomplish LE4 of the substrate.

Most of these arriving electrons 37 will collide with sample 16, give upsome of their kinetic energy to it, and be reflected away into the bodyof the plasma. Some of them will stick to the substrate surface, therebytransferring negative charge to it. If sample 16 is an insulator, itcannot drain off this surface charge, which could accumulatesufficiently to generate a negative potential capable of blocking thearrival of further electrons 37, thereby stopping the etching process.To prevent this, sample 16 must be periodically “discharged” or“neutralized” by applying a periodic ac external bias to it onconnection 18 using AC power source 19. During the positive swing ofthis bias, electrons 37 will be attracted to sample 16, and LE4 willoccur; during the negative swing, accumulated negative surface chargewill be ejected, in effect neutralizing the substrate surface.

Applying a positive bias to a plasma source, relative to additionalstructure 14, in contact with the plasma, enables the extraction of abeam of electrons from the plasma. The present invention extracts a beamof electrons from the plasma. Alternatively, the electron extractionbias on the ac plasma source could be modulated, so that during itspositive swing, positive ions (instead of electrons) would be extractedfrom the source and beamed to the substrate, effecting neutralization ofthe accumulated negative charge. Magnitude of the positive bias would bekept too low to give ions sufficient energy for ion bombardment damageat the substrate.

Referring now to FIG. 2, in a variation of this third embodiment, andapplicable in a dc plasma environment for etching conducting substrates,sample 16 is placed upon anode 51 of dc plasma reactor 50 in similarfashion to that described in commonly assigned U.S. Pat. No. 5,882,538,filed Aug. 28, 1996 and titled “METHOD AND APPARATUS FOR LOW ENERGYELECTRON ENHANCED ETCHING OF SUBSTRATES”, however as an improvementthereon, additional structure 14 as described above is placed withinplasma chamber 11 in close proximity to sample 16. By electricallybiasing additional structure 14 using AC power source 21 and DC powersource 28 as described above, improved precise control over the flux andenergy of charged species being imparted to sample 16 is achieved. Inaddition to anode 51, dc plasma reactor 50 includes cathode 56 and dcpower source 54. DC power source 54 is connected to cathode 56 on line57 and to anode 51 on line 58. In addition anode 51 is attached toground 59. The operation of the remaining components of FIG. 2 issimilar to that as described with respect to FIG. 1.

FIG. 3 is illustrative of the present invention in a dcplasma-generating environment and illustrates a dc plasma reactor 60employing the concepts of the present invention. DC power source 54generates a dc plasma in plasma chamber 11 using cathode 56 and anode 51as known by those skilled in the art and will not be explained indetail. The operation of the invention is the same as with respect toFIG. 1.

Cathode 56 is mounted in the plasma chamber 11. Cathode 56 is preferablya cold cathode, sometimes referred to as a field emission cathode,meaning it functions without the application of heat. The cathode isactivated using external power source 54 that applies a direct current(DC) voltage between cathode 56 and anode 51. Because the chambercontains a gas, the chamber functions as a DC glow-discharge tube or DCplasma reactor. The cathode may be a standard cold cathode such as areknown in the art or one constructed in accordance with the teaching ofcommonly assigned U.S. Pat. No. 5,917,285, filed Jul. 23, 1997, titled“APPARATUS AND METHOD FOR REDUCING OPERATING VOLTAGE IN GAS DISCHARGEDEVICES”.

FIG. 4 is a view illustrating an ac plasma reactor 70 employing theconcepts of the present invention. AC plasma reactor 70 illustrates aninductively coupled plasma reactor and is merely one type of ac plasmareactor. The present invention is equally applicable to any ac plasmareactor, the ac reactor shown in FIG. 4 being used for illustrativepurposes only. AC power source 71 supplies power to coil 72 throughconnections 74 and 76. AC power source 71 and coil 72 create an acplasma within plasma chamber 11 as known by those skilled in the art andwill not be explained in detail. The operation of the invention is thesame as with respect to FIG. 1.

FIG. 5 is a detail schematic view of sample support apparatus 80, whichincludes the mechanical support 12 and additional structure 14 of thepresent invention. Additional structure 14, connected to AC and DC powersources on connection 17 and capable of imparting an electricalpotential, is placed within plasma chamber 11 (omitted for clarity) inclose proximity to sample 16. Additional structure 14 can be fabricatedin a manner similar to that described with respect to the cold cathodediscussed above and is permeable in order to promote the unimpededpassage of charged and neutral species.

FIG. 5 a is a detail schematic view of sample support apparatus 80,which includes the mechanical support 12 and additional structure 14 ofthe present invention. Additional structure 14, connected to AC and DCpower sources on connection 17 and capable of imparting an electricalpotential, is placed within plasma chamber 11 (omitted for clarity).Additional structure 14 can be fabricated in a manner similar to thatdescribed with respect to the cold cathode discussed above and ispermeable in order to promote the unimpeded passage of charged andneutral species. In this embodiment, insulating non-permeable ring 23 isplaced upon mechanical support 12 in such a way as to surround sample16. Ring 23 is joined to mechanical support 12 and additional structure14 in such a way as to prevent electrons and ions from penetratingjoints 13 and 13 a. This arrangement ensures that any electrons or ionsthat reach sample 16 must pass through additional structure 14.

It will be obvious to those skilled in the art that many modificationsmay be made to the preferred embodiments of the present invention, asset forth above, without departing substantially from the principles ofthe present invention. All such modifications are intended to beincluded herein within the scope of the present invention, as defined inthe following claims.

1. An apparatus for low-damage anisotropic low energy electron enhancedetching of a substrate, comprising: a plasma reactor; plasma creationmeans at least partially disposed within the plasma reactor for creatinga plasma having positively charged ions and electrons; a substrateholder disposed within the plasma reactor for receiving a substrate,wherein the substrate holder is isolated from the plasma creation means;electron etcher means for etching material from the substrate withelectrons from the plasma, wherein the electron etching means is inelectrical communication with the substrate holder; and charged particlecontroller means, disposed proximal to the substrate holder, forcontrolling the flux of charged particles directed from the plasma ontoa substrate disposed on the substrate holder, the flux having sufficientenergy for the electrons to etch material from the substrate.
 2. Theapparatus of claim 1, wherein the charged particle controller means isadapted to control the energy of charged particles being impacted ontothe substrate.
 3. The apparatus of claim 1, further including: a chargedparticle blocking means for preventing charged particles in the plasmafrom reaching the substrate unless the charged particles pass throughthe charged particle controller means.
 4. The apparatus of claim 1,further comprising: a pulse waveform power source adapted toelectrically bias the charged particle controller means to direct theelectrons from the plasma towards the substrate.
 5. The apparatus ofclaim 4, wherein said pulse waveform power source is further adapted toperiodically bias the charged particle controller means to direct ionsfrom the plasma towards the substrate to electrically neutralize thesubstrate.
 6. The apparatus of claim 4, wherein said pulse waveformpower source is adapted to cycle between a positive electrical potentialand a negative electrical potential, and wherein the positive potentialis such that electrons having kinetic energy less than 100electron-volts are attracted to the substrate and etch materialtherefrom.
 7. The apparatus of claim 6, wherein a waveform of a pulsewave supplied by the pulse waveform power source is defined by a periodhaving a first predetermined interval at the positive electricalpotential and a second predetermined interval at the negative electricalpotential, wherein during the first interval electrons are directed tothe substrate, and wherein the second interval is of duration such thata sufficient number of ions are directed to the substrate tosubstantially neutralize the accumulated electrons on the substrate. 8.The apparatus of claim 6, wherein a waveform of a pulse wave supplied bythe pulse waveform power source is defined by a period having a firstpredetermined interval at the positive electrical potential and a secondpredetermined interval at the negative electrical potential, whereinduring the first interval electrons are directed to the substrate toetch the material from the substrate, and wherein the second interval isof duration such that a sufficient number of ions are directed to thesubstrate to substantially neutralize the accumulated electrons on thesubstrate.